Recently, there has been increased interest in the development of anion exchange membrane fuel cells (AEMFCs). The fundamental difference between AEMFCs and the more widely studied proton exchange membrane fuel cells is that the former operate at high pH thus requiring the membrane to conduct hydroxide ions from the cathode to the anode. The key advantage of operating a fuel cell under alkaline conditions is the potential to forgo noble metal catalysts due to the low overpotentials associated with many electrochemical reactions at high pH. The improved electrokinetics also allow for the possible use of high energy density fuels such as ethanol which is also a renewable resource as it can be produced directly by fermentation of biomass. A major challenge in the development of AEMFCs is the need for an anion exchange membrane (AEM) that is chemically stable under the conditions within an AEMFC.
AEMs are typically made with polymers that have pendant cationic groups. By far the most commonly reported cationic group is the benzyl trimethylammonium (BTMA) cation. AEMs have been prepared with BTMA cations attached to polymer backbones such as poly(phenylene), poly(tetrafluoroethene-co-hexafluoropropylene), poly(phenylene oxide), poly(ether-imide), poly(arylene ether sulfone), and poly(ether ether ketone).
Many of these BTMA-containing membranes are reported to have good chemical stability. For example, the ion exchange capacity of a radiation-grafted perfluorinated AEM with BTMA cations was shown to decrease by less than 5% after a 233-hour fuel cell test at 50° C. Another study of the degradation mechanisms of tetraalkylammonium compounds concluded that maintaining hydration around the cations is critical to stability and that, under the correct conditions, such cations possess reasonable stability at temperatures above 60° C. Despite reports such as this, BTMA cations are generally considered to have insufficient stability for long-term use in AEMFCs. Thus the investigation of cationic groups with improved chemical stability is of paramount importance to the development of AEMFCs.
One relatively early study of cation stabilities found that quaternized 4,4′-diazobicyclo-[2.2.2]-octane cations had improved stability to alkaline conditions when compared to BTMA cations. Another approach to preparing more stable cations is to reduce susceptibility to nucleophilic attack by using resonance-stabilized cations such as guanidinium or imidazolium groups. Other reports have included the use of coordinated metal cations or phosphonium cations with bulky electron-donating substituents to both sterically protect the ion from nucleophilic attack and to lessen the charge density on the phosphorous atom. Additionally, it has been reported that attachment of quaternary ammonium groups to the polymer backbone via an alkylene spacer of >3 carbon atoms can lead to improved chemical stability. Attachment of imidazolium and guanidinium groups with alkylene spacers have also been reported.
We have previously described the preparation of AEMs with BTMA cations on a poly(phenylene) backbone that is very stable under alkaline conditions and in AEMFC testing.